The aim of this revised project is to understand more about BRCA1 function and its relation to p53. The tumor suppressor gene p53 is a transcriptional factor that has been implicated in the cellular response to DNA damage and mediates either growth arrest/senescence or apoptosis. The tumor suppressor BRCA1 is less characterized since there is no BRCA1 - null cell line available, but over-expression of BRCA1 has been shown to lead to growth arrest or apoptosis. Our preliminary data indicate that BRCA1 is transcriptionally down- regulated by wt-p53 induction (ectopically and in response to DNA damage). However, BRCA1 basal level of expression is higher in p53 null mammary epithelial cells isolated from p53 knock-out mice than in the corresponding wt-p53 cells. Moreover, those p53 knock-out cells show an opposite response to DNA damage, indicating that BRCA1 mRNA is induced rather than repressed following DNA damage. In this application, we propose to investigate the hypothesis that wt- p53 can be a negative regulator of BRCA1 . We will determine whether re-expression of BRCA1 in a BRCA1 repressed context (p53 induced system) will overcome the p53-mediated cell growth arrest. We will also generate wt- or mutant-p53 knock-in cells to further characterize the negative correlation between p53 and BRCA1. We will analyze the molecular basis of transcriptional repression of the BRCA1 promoter by p53 by further localizing the p53 responsive region of the BRCA1 promoter. A variety of p53 mutants (naturally occurring hot spot mutations as well as recombinant mutants) will be analyzed in an effort to identify and define p53 determinants required for BRCA1 repression. The long term goals of this project are to improve our understanding of the functional role of the BRCA1 gene in breast cancer development.